Surgical tourniquet systems are widely used to facilitate surgical procedures on portions of arms or legs of patients by occluding blood flow into those limb portions for periods ranging from a few minutes to several hours.
A surgical tourniquet system at present typically includes an inflatable cuff for encircling the limb and for applying pressure to the encircled limb, pressure regulating means intended for maintaining the pressure applied by the cuff near a desired pressure, and means for setting the desired pressure, either at a constant level or at a level which varies in response to a changing physiological parameter such as systolic mean or diastolic blood pressure.
In principle, it is considered safest to maintain the cuff pressure near the "minimum effective pressure", i.e. the lowest pressure that will occlude blood flow past the cuff. In practice, the minimum effective pressure is difficult to predict and achieve because it is a function of many variables, including the patient's changing blood pressure, the patient's limb size and tissue characteristics, the relative match of cuff shape to limb shape, cuff characteristics such as its width, and the technique of cuff application to the limb.
A related safety issue concerns the pressure distribution or pressure gradient generated beneath an encircling cuff. The results of tests performed on animals, cadavers and to some extent humans, as described in the cited prior art, clearly indicate that high gradients of pressure applied to nerve tissues beneath tourniquet cuffs are associated with higher probabilities of nerve injury. Conversely, lower pressure gradients are associated with lower probabilities of nerve injury. These findings would appear to have important implications for surgical tourniquet systems, but surprisingly no tourniquet system known to the applicant in the prior art permits the estimation and regulation of the pressure actually applied by the cuff to the limb near one or more predetermined locations relative to the cuff.
At present, all commercially available tourniquet systems employ pneumatic cuffs and pneumatic pressure regulating mechanisms. These systems monitor and maintain the pressure in an inflatable bladder in the cuff, but do not estimate the pressure actually applied by the cuff to the limb at the cuff/limb interface. For example, in the extreme, one could envision a cuff completely detached from a limb, and thus completely ineffective, in which the bladder pressure continues to be maintained precisely near a desired level.
Recently, research and clinical investigations were performed using a tourniquet cuff incorporating a novel biomedical pressure transducer, described in my co-pending United States patent application having serial number 07/033,770 and filed Apr. 3, 1987. This application having serial number 07/033,770 is herein incorporated by reference. The clinical investigations performed using the novel biomedical pressure transducer have revealed many significant discrepancies between the pressure maintained in the inflatable chamber of a pneumatic tourniquet cuff and the pressure actually applied by the cuff to the encircled limb. For example, with the transducer oriented to estimate applied pressure near a plurality of predetermined cuff locations in an axial direction from the proximal to the distal edge of the cuff, it was found that the pressure varied from a level of almost zero near the edges to a maximum level near the cuff center, and it was found that the maximum level differed significantly from the pressure in the cuff bladder as a function primarily of how snugly the cuff had been wrapped around the limb. Normal variations in snugness were found to vary the maximum pressure actually applied to the limb by 50 percent or more in comparison to the regulated bladder pressure; such a discrepancy is undesirable clinically, and in many clinical situations, such as in the use of intravenous regional anesthesia, such a discrepancy can present a life-threatening hazard.
Further tests with a tourniquet cuff including the above-referenced biomedical pressure transducer have revealed significant pressure variations in the pressure applied to a limb in a circumferential direction around the limb, particularly in the region where the cuff overlaps itself as it encircles the limb. Such variations can be hazardous.
Tests of a variety of pneumatic tourniquet cuffs and non-pneumatic occlusive bands which have included the above referenced transducer but which have had different designs, as well as tests of each such cuff on a variety of limb shapes, have shown that a wide variety of pressure gradients can be applied to the encircled limb and to the underlying nerve tissues.
No surgical tourniquet system in the prior art known to the applicant accurately estimates the level of pressure actually applied to the limb near a predetermined cuff location, when the specific cuff of that system is applied with an arbitrary degree of snugness to a limb of arbitrary shape. Moreover, the applicant is unaware of any surgical tourniquet system known in the prior art which can accurately estimate the axial pressure distribution, the circumferential pressure distribution, or the maximum pressure actually applied to the limb in view of the above described variables. Finally, and accordingly, no tourniquet system is known in the prior art which can regulate the maximum pressure actually applied by the cuff to the limb which it encircles, or the pressure applied near one cuff location, or the pressures actually applied to the limb near multiple cuff locations in relation to desired pressure levels.
An object of the present invention is to provide a tourniquet which is capable of indicating a pressure actually applied by a cuff to a limb near any predetermined location beneath the cuff, in a manner which continues to be accurate despite differences in the initial snugness of cuff application on different limbs, and despite considerable intra-operative changes in the degree of snugness and shape match between the cuff and limb as a result of limb manipulation in surgery, for example. A related object of the invention is to provide a tourniquet capable of accurately regulating a pressure actually applied by a cuff to a limb near a predetermined location relative to the cuff.
A further object of the present invention is to provide a tourniquet capable of indicating multiple pressures actually applied by a cuff to a limb near multiple predetermined locations beneath the cuff. A related object is to provide a tourniquet capable of producing a clinically desired distribution or gradient of pressures on the limb beneath the cuff by, for example, making changes in the design of the cuff which change the applied pressures appropriately. Another related object of the invention is to provide a tourniquet which is capable of separately regulating the pressures applied by the cuff to the limb near multiple predetermined locations beneath the cuff, so that clinically desired distributions or gradients of pressures in circumferential and axial directions can be maintained for an extended period of time. A further related object is to provide a tourniquet in which the distribution or gradient of pressures applied by the cuff to the limb can be conveniently specified and changed from time to time by a clinical operator of the tourniquet.
The applicant is aware of the following United States patents which are more or less relevant to the subject matter of the applicant's invention.
______________________________________ 4,770,175 9/1988 McEwen 128/327 4,605,010 8/1986 McEwen 128/686 4,479,494 10/1984 McEwen 128/327 128/682 4,469,099 9/1984 McEwen 128/327 128/682 ______________________________________
The applicant is also aware of the following published references which are more or less relevant to the subject matter of the applicant's invention.
J. A. McEwen and R. W. McGraw, "An adaptive tourniquet for improved safety in surgery." IEEE Transactions in Biomedical Engineering, Vol.BME-29, February 1982, pp. 122-128.
J. A. McEwen and G. F. Auchinleck, "Advances in surgical tourniquets." J. Assn. Operating Room Nurses, Vol. 36, 1982, pp. 889-896.
J. A. Shaw and D. G. Murray, "The relationship between tourniquet pressure and underlying soft-tissue pressure in the thigh." The Journal of Bone and Joint Surgery, Vol. 64-A, 1982, pp. 1148-1152.
A. C. McLaren and C. H. Rorabeck, "The pressure distribution under tourniquets." The Journal of Bone and Joint Surgery, Vol. 67-A, 1985, pp. 433-438.
R. J. Newman and A. Muirhead, "A safe and effective low pressure tourniquet." Journal of Bone and Joint Surgery, Vol. 68-B, 1986, pp. 625-628.
J. A. Shaw, W. W. Demuth, and A. W. Gillespy, "Guidelines for the use of digital tourniquets based on physiological pressure measurements." The Journal of Bone and Joint Surgery, Vol. 67-A, 1985, pp. 1086-1090.
S. E. Grice et al., "Intravenous regional anesthesia: Evaluation and prevention of leakage under the tourniquet." Anesthesiology, Vol. 65, pp. 316-320, 1986.